Tungsten ores, such as scheelite or wolframite, used for the production of tungsten and tungsten chemicals invariably contain several weight percent (w/o) of molybdenum. The presence of molybdenum in the final product is undesirable because many exacting applications of tungsten metal and tungsten chemicals require materials of high purity; the molybdenum impurity affects such materials deleteriously. However, the separation of molybdenum from tungsten is very difficult because these two elements have analogous outer shell electronic configurations and, therefore, their chemical properties are very similar.
The prevalent commercial method of processing of tungsten ore or tungsten ore concentrate consists of the digestion of the feedstock in strong alkali such as sodium hydroxide. The removal of the molybdenum impurity is accomplished by the precipitation of the molybdenum sulfide which requires the addition of sulfide ion and acidification of the process stream to a pH of 3. This method has the disadvantage of generating large amounts of a sludge precipitate which is difficult to reprocess or to dispose of. The technique results in a significant loss of tungsten because a comparable weight of tungsten is either entrapped or co-precipitated during the molybdenum sulfide precipitation. In addition, the operation involves the use of environmentally objectionable sulfide compounds (the generation of hydrogen sulfide is unavoidable) and large amounts of acid.